In general, an electrophotographic method includes uniform charging of an inorganic photoconductive substance (photosensitive drum) such as selenium, amorphous silicon and zinc oxide, or an organic photoconductive substance (photosensitive drum) such as diazo compound, pigment, etc., which has been in most cases prepared into a drum, irradiation of an image modulated light to form an electrostatic latent image, said image being developed by allowing a powder material to be attracted by static electricity, transferring the powder material on a receiving surface such as paper, film, etc. as necessary, and fixing same by applying pressure, heat, and so on. The electrophotographic method has been widely used for copier, laser printer, facsimile, etc.
The powder for forming an image by developing the electrostatic latent image on the photosensitive drum, which is ultimately transferred onto a receiving surface such as paper or film in the electrophotographic method is referred to as a toner. A toner is usually mixed with a carrier such as glass beads, iron powder, ferrite, etc., and used as a developing powder. As the toner as a developing powder for electrophotography, used are particles prepared by the pulverizing method comprising mixing and kneading of a binder resin with colorants, charge control agents, flowability improvers, pulverization aids, etc., pulverizing the obtained mixture, and classifying the same.
As the toner binder resin, styrene/acrylic copolymers have been mainly used. However, polyester resins have been drawing attention recently in view of their excellent fixability at low temperatures as demanded by the increased speed and coloring of electrophotography.
In particular, colored electrophotography requires gloss on the image surface from the aspect of color reproduction, and polyester resins which afford superior surface gloss by low temperature fixing have been increasingly used.
The polyester resins conventionally used are mainly unsaturated polyester resins obtained by condensation polymerization of aliphatic unsaturated carboxylic acids such as fumalic acid, maleic acid, etc. with diols having bisphenol structure.
The glass transition temperature of the polyester resin depends mainly on number average molecular weight. The unsaturated polyester resin is generally polymerized by the normal pressure method, whereas a high molecular weight polyester resin cannot be produced by the production processes thereof, thus failing to achieve high glass transition temperature. The glass transition temperature of unsaturated polyester resin obtained by conventional methods is about 55.degree. C., which in turn causes poor storage stability of a toner comprising an unsaturated polyester resin as a binder resin, despite its superior low temperature fixing characteristics, as evidenced by the fact that a long-term storage at high temperature results in blocking of the toner.
In case where bisphenol diols are used, plasticizers used in sheets made of vinyl chloride resins and erasers tend to transfer. For this reason, when copied images are kept in a clear file or on a desk mat made of vinyl chloride resins, or an eraser or eraser refuse is left on the images, the plasticizer contained in the vinyl chloride sheet or eraser gives rise to the damaged images and staining of the clear sheet or eraser.
In the case of colored electrophotography, it is required that a toner should possess high transparency from the aspects of reproduction of intermediate colors and penetrability of the sheets for overhead projectors. The use of a dye as a colorant is preferable for increasing transparency of a toner. However, resins comprising bisphenol type diols do not permit sufficient color production by the dye, since they cause marked degradation of color fastness to heat and color fastness to light of the dye, for which reason the colorant is limited to pigments which are poor in transparency. In order to produce transparent toners by using pigments, it is necessary to disperse pigments finely pulverized to the size smaller than the light wavelength in a resin as primary particles, and this process gives rise to various problems while processing.
There has been an attempt to use aromatic polyester resins obtained from aromatic dicarboxylic acids such as terephthalic acid and isophthalic acid, and aliphatic diols such as ethylene glycol and neopentyl glycol as a toner binder, besides the above-mentioned unsaturated polyester resins.
The aromatic polyester resins can be easily made to have high molecular weight, since they are usually polymerized by the reduced pressure method. For this reason, high glass transition temperature of the aromatic polyester resins can be achieved rather easily, and can afford toners with good storage stability.
On the other hand, practical glass transition temperature of about 58.degree. C. or more, preferably 60 .degree. C. or more, more preferably 63.degree. C. or more requires extremely high number average molecular weight of polyester resin, which in turn raises melt viscosity of the polyester resin, impairing low temperature fixability possessed by the polyester resin.
For the realization of high glass transition temperature while retaining low temperature fixability, there has been proposed introduction of bisphenol type diols as a diol component. When the bisphenol type diols are introduced, both the glass transition temperature of not less than about 58.degree. C. and low temperature fixability can be obtained. In this case, however, the same problems as in the case of the aforementioned unsaturated polyester resin, namely, decrease in color fastness to heat, color fastness to light, and resistance to plasticizer can occur, and a toner for electrophotography having good characteristics cannot be obtained.